1. Field of the Invention
The invention relates to power amplifiers for broad band high power operation at frequencies above one GHZ and constructed using MMIC fabrication techniques.
2. Prior Art
The term "Monolithic Microwave Integrated Circuit" (MMIC) implies a compact circuit which performs at microwave frequencies, and which uses a fabrication technique in which passive and active components are formed in place upon a substrate of a monocrystalline material, such as gallium arsenide, convertible to a semiconductor. Such circuits achieve performance at frequencies at 1 GHZ and above, which is not easily matched in circuits using discrete components. The discrete components, though miniaturized, must usually be interconnected by leaded connections which form unassimilable reactances at such frequencies. The MMIC designs avoid discrete components, and instead form them in place on an insulating region of the substrate. The resulting MMIC arrangement is compact and compactness leads to efficiency. The substrate connections frequently become true transmission lines and the dimensions which affect electrical properties, are selected to set impedance levels, reactances, and resonances.
Both signal gain and power transfer must be optimized in a MMIC power amplifier. The efficient transfer of power from a generator to a load requires matching the impedance of the load to the internal impedance of the generator. The standard for RF signal networks remains 50 ohms even at microwave frequencies and the standard has unquestioned practical advantages. Thus even at gigahertz frequencies-, a power amplifier will be expected to have RF signal voltage gain, and to match the 50 ohm transmission line from which it derives its input signal and the 50 ohm transmission line into which it couples its output signal. (The impedance match implies the coupling of RF power from generator to load with minimum reflection (i.e. low S11, S22 parameters).)
On a MMIC, microstrip transmission lines readily match higher impedances than 50 ohms at low power levels but matching lower impedances at high power levels requires care in design to maintain efficient MMIC compactness. When higher power levels are being considered, RF current levels on the order of an ampere appear, and the requirement placed on the transmission line is to match such lower impedances. For instance, when the active power devices are high frequency transistors, whose operating voltage is on the order of 10 volts, the "generator" impedances are on the order of an ohm. The narrow width printed circuit runs required for compactness of MMIC designs makes the attainment of circuit runs at one ohm impedance levels awkward, and they must be used sparingly if at all. Furthermore, efficient power matching through this range of impedance levels is difficult and impractical except for narrow band operation.
On the other hand, transistors operating at GHZ frequencies have problems of their own for high power operation The efficient distribution of RF energy through the devices becomes difficult as the power level/current level increases, and the lengths of the fingers defining the active region increase. It becomes evident that operation of several devices in parallel may be required as opposed to operation of one large device. In addition, optimization within the device of the manifolding structure (by means of which internal parallelization is achieved), may also be required.
From the above considerations, it is clear that achieving high power-high bandwidth operation of MMIC power amplifiers requires care in selection of the topology and electrical design of the active power devices as well as care in selection of the topology and electrical design of the accompanying circuitry. The present invention is addressed to achieving high power by careful selection in respect to both the active device employed and the passive circuitry by means of which they are connected together and to the external world.
A major application of MMICs is in conjunction with solid state radar systems in which, in current designs, a single solid state power stage drives a single element of the array. The problem posed for the solid state design is in achieving the high power levels that prior designs have achieved and in addition, achieving wide band operation at these high power levels. The requirements of high power and high bandwidth conflict, and while narrow band designs have achieved quite high power levels, wide band designs have not.